France has said the majority of Russia's air strikes inside Syria have been aimed at supporting president Bashar al-Assad's forces and not targeting Islamic State (IS) militants as Moscow claims.

French defence minister Jean-Yves Le Drian has criticised Russia's recent involvement in the conflict, saying that "80 to 90 per cent" of its air strikes were in support of Syrian president Bashar al-Assad's regime, Moscow's long-term ally.

"The Russian military action in Syria over the past 10 days does not target Daesh, their main aim is [to ensure] the security of Bashar al-Assad," Mr Le Drian said.

"We do not consider Bashar part of the solution."

In a second wave of French airstrikes inside Syria, two Rafale jets dropped bombs on an IS training camp, with Mr Le Drian saying the "objectives were accomplished" and vowing more attacks will follow.

As with a first wave of strikes on September 27, the attacks focused on the IS stronghold of Raqqa in northern Syria.

"We know that in Syria, in particular around Raqqa, there are training camps for foreign combatants whose mission is not to go fight for IS in the Levant but to come to France, to Europe, to carry out attacks," Mr Le Drian said.

Mr Carter said the new approach focused more on enabling forces already on the ground to battle IS. The original US military effort sought to train entire units outside Syria, at sites in Turkey and Jordan, and then send them back into the civil war.

"We have devised a number of different approaches," Mr Carter said in London.

Meanwhile, the IS group advanced to the closest it has been to the Syrian city of Aleppo at dawn on Friday, after hours of ferocious fighting, the Britain-based Syrian Observatory for Human Rights reported.

"Dozens of combatants were killed on both sides," director Rami Abdelrahman said.

After a night of fierce clashes, IS militants had driven out rebels from the localities of Tall Qrah, Tall Soussin, Kafar Qares and the base of Madrasat al-Mushat by early morning, he said.

The seizure of these positions brought the jihadists to about 20 kilometres from the front line where forces loyal to Mr Assad are positioned, including the Sheikh Najjar industrial zone.

"IS has never been so close to the city of Aleppo, and this is its biggest advance towards" the country's pre-war commercial capital, Mr Abdelrahman said, whose Britain-based group relies on a network of sources on the ground across Syria.

The IS group claimed territory mostly to the north-east of Aleppo, where it controls towns and regions including Al-Bab, one of its strongholds.

The conflict began as an uprising against Assad's rule in 2011 but has splintered into a multi-faceted civil war involving government troops, Western-backed rebels, jihadists and Kurdish forces.

More than 240,000 people have been killed, with four million more forced to flee the country.

Iranian general killed in Syria: officials

Meanwhile, an Iranian Revolutionary Guards general has been killed near Aleppo, where he was advising the Syrian army on their battle against IS fighters, the guards said in a statement.

General Hossein Hamedani was killed late on Thursday, the statement said.

Iran is the main regional ally of Mr Assad, and has provided military and economic support during Syria's four-year-old civil war.

In the biggest deployment of Iranian forces yet, sources told Reuters last week that hundreds of troops had arrived since late September to take part in a major ground offensive planned in west and north-west Syria.